Order content documents in the EPUB
spine to ensure the reading order is logical and
identify whether each document contains linear or non-linear content.
- Order the entries for all content documents.
- Use the
linearattribute to differentiate linear and non-linear content.
Frequently Asked Questions
- Do reading systems hide non-linear content from readers when paging through a publication?
No, the EPUB specification does not mandate a specific way of rendering non-linear content, so you cannot rely on reading systems hiding content marked as non-linear. If you place a non-linear document between two chapters, some users will encounter that content as they manually page through the publication.
Although some reading systems will ignore non-linear documents, only rendering them if a user clicks on a link to them, such behaviour is not universal.
spine element in the
EPUB package document defines the order of documents in the publication. Reading systems use
the element to determine which document comes next when users are reading.
spine consists of one or more
itemref elements, each of which
references an item in the manifest.
idref attribute on the
itemref provides the specific ID of manifest
item to render, as depicted in the following example. (Note there is no hash (#) at the start of
the reference as the value is an ID reference not a URL.)
<package …> … <manifest> … <item id="c01" href="xhtml/chapter01.xhtml" media-type="application/xhtml+xml"/> … </manifest> <spine> … <itemref idref="c01"/> … </spine> </package>
The average reader is typically not aware that an EPUB publication is usually a set of many
documents in the spine. The reading system hides this fact from them, as when they reach the
end of one document, it automatically propagates the next page by inspecting the
spine to load the next.
It is critical that you reference all the primary content documents from the
and you must order these documents to present the primary narrative of the publication in
logical sequence. Although misorderding documents is not common, it can happen during edits and
revisions (e.g., if you insert a new chapter or section). In these cases, all readers will be
confused if the documents are out of sequence, or if one is missing, so it is more of a general
usability issue than specifically an accessibility one.
Linear content documents are those that the author expects users to read sequentially: Chapter 1 is followed by Chapter 2, which is followed by Chapter 3, and so on. The linear reading order represents the primary pathway through a publication.
If readers are expected to read linear documents in sequence, then it follows that they are not required to read non-linear documents that way. An answer key is an example of a non-linear document. The reader might want to review it after the chapter containing the corresponding quiz, they might want to wait until the end of the book before checking the answers, or they might not be interested in it at all. It is not essential to read the answers where the author has inserted them into the spine, in other words.
Non-linear content documents are identified by adding a
linear attribute with the
no to the
itemref element, as depicted in the following
<itemref idref="answers-01" linear="no"/>
Reading systems assume that
itemref elements without a
attribute represent linear content documents, so it is rare to find
linear="yes" set in EPUB publications.
The placement of non-linear content in the
spine does pose some issues for the
readability of the content. Because reading systems do not all handle non-linear content the
same way, authors cannot assume how reading systems will present it to users. Reading systems
may or may not present such content where it is placed in the
Consequently, a primary consideration for non-linear content is to ensure that its nature and purpose is clear on its own. Do not assume that readers will have followed a link to reach the non-linear content, so will already know its purpose.
Using the example of an answer key, make sure that the file starts with a heading that identifies that what follows are the answers to the quiz. If you place image descriptions in a separate non-linear file, ensure that each description makes clear what image it is describing.
Another option for non-linear content is to place it where it will have the least impact on the
linear reading experience. You can place image descriptions and pop-out images after all the
linear content in the spine, for example (see example 2). Placement at the
end of the
spine is particularly useful when you cannot adequately describe the
content. An enlarged version of an image might suffer from an explicit caption, for
EPUB 3's requirements for non-linear content differ from EPUB 2's in one significant way, namely authors must include a link to all non-linear documents in the spine. This requirement ensures that readers can reach the non-linear content regardless of whether reading systems render it while manually paging through a publication.
In most cases, this requirement is not onerous to meet, as authors typically do not include
non-linear content that they have not linked from the linear content. One known issue
involves including a non-linear cover in the spine (see example 1). To
satisfy the linking requirement to this document, you can include a link to the cover in